1. Technical Field
The present invention relates in general to an improved bearing for a disk drive, and in particular to an improved fluid journal bearing for a disk drive spindle.
2. Description of the Related Art
Generally, a data access and storage system consists of one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, two or three disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm).
The only other moving part within a typical HDD is the actuator assembly. The actuator moves magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
Typically, a slider is formed with an aerodynamic pattern of protrusions (air bearing design) on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive. In general, a slider is associated with each side of each platter and flies just over the platter""s surface. Each slider is mounted on a suspension to form a head gimbal assembly (HGA). The HGA is then attached to a semi-rigid actuator arm that supports the entire head flying unit. Several semi-rigid arms may be combined to form a single movable unit having either a linear bearing or a rotary pivotal bearing system.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk.
When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop directly over the desired track.
In the past, the spindle bearing system in a disk drive has typically used ball bearings. This type of rotary bearing system produces non-repeatable radial runout (NRRO) motion of the disk that adversely affects the ability of the actuator to precisely locate and position the slider and magnetic read/write head over the data track on the disk. As disk drives are implementing ever-increasing track densities, or reduced spacing track to track, it is desirable to replace ball bearing spindles with fluid dynamic bearing (FDB) spindles that have essentially no NRRO motion. In a hydrodynamic FDB spindle, the rotating journal sleeve rotates very precisely about a stationary cylinder on a thin pressurized fluid film much in the same way the slider flies over the disk. This fluid may be air or any other appropriate viscous fluid such as oil.
One type of journal bearing makes use of ferromagnetic materials, such as 440 stainless steel, in the bearing surfaces. For example, FIGS. 2 and 3 illustrate a sectional side elevation and sectional end view of a conventional fluid bearing spindle 11 in a disk drive. Spindle 11 has a stationary shaft 13 and a sleeve 15. A small fluid bearing gap 16, typically air, is formed between sleeve 15 and shaft 13 (on the order of a few microns), such that sleeve 15 is rotatable with respect to shaft 13. A ferromagnetic hub 17 is bound to sleeve 15, such as by shrink fit or adhesive bond. In addition, rotor magnets 19 (alternately radially poled) are bound to an exterior of hub 17, which are spaced apart from a circumferential stator/windings 21. The large curved arrows 23 represent return flux paths from magnets 19.
Even though the fluid medium of bearing gap 16 has a magnetic permeability that is thousands of times greater than that of the materials used to form shaft 13 and sleeve 15, the reluctance of gap 16 is on the same order of magnitude as shaft 13 or sleeve 15 because gap 16 is so small. As a result, the magnetic flux in rotating hub 17 and sleeve 15 can leak into stationary shaft 13, resulting in a large braking effect and, thus, substantial iron loss in shaft 13. Thus, an improved disk drive spindle design which overcomes these problems is needed.
One embodiment of a spindle design for a disk drive actuator assembly constructed in accordance with the present invention includes a significantly large radial gap between the rotating, ferromagnetic hub and rotating sleeve of the fluid bearing spindle. The large gap may be filled with a medium, such as air, or any other non-permeable material. The large gap is preferably on the order of several hundred microns. Because of the large gap, the magnetic flux leakage from the rotating journal sleeve into the stationary shaft at the center of the spindle is negligible. Consequently, iron loss in the shaft caused by magnetic flux leakage into the shaft is reduced to acceptable noise levels.
The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the preferred embodiment of the present invention, taken in conjunction with the appended claims and the accompanying drawings.